Evaluation of various data modulation schemes!
The digital data (original information) normally handled by us have a signal format with 0s and 1s aligned in an arbitrary sequence. When considering the characteristics of the recording and reproduction systems, however, this format in its direct form cannot be efficiently used as recording signals. In common practice, therefore, the original information is converted (modulated) into a form suitable for the above-mentioned recording media and recorded, The signal reproduced from the recording medium is restored (demodulated) to the original information. In recent years, various recording codes have been suggested for digital data recording schemes. The characteristics requirements for modulation-demodulation for data in an optical disk, for example, are as follows:
(1) In order to increase the file capacity, a high density is possible. PA0 (2) A self-synchronism is easy for retrieving the information signal from the reproduced signal. PA0 (3) In order to prevent any read error due to fluctuations of a reference level in digitization at the time of reproduction or in order to prevent fluctuations of the servo error signal at the time of recording or reproduction, the frequency spectrum of the modulated signal is DC free (contains no DC components). PA0 (4) Propagation of a bit error is difficult to affect the reproduction of the successively arriving bit information
Familiar modulation schemes for recording data on an optical disk include NRZ, NRZI, FM, EFM and (2, 7 )-RLL schemes. The number of the same successive bits in the data sequence is called the Run.
The NRZ (Non Return to Zero) scheme is often used for magnetic recording of a hard disk. In this scheme, signal levels for logical values 0 and 1 of digital data exactly correspond to low and high signal levels, respectively. This scheme, therefore, is most easy to understand for conversion of an electrical signal for digital data.
The NRZI (Non Return to Zero Inverse) scheme, on the other hand, is one in which the signal level is inverted only when the logical value of the digital data is 1.
In the NRZ and NRZI schemes, the maximum inversion width Tmax of the data string is infinitely large, and it is understood that self-synchronization is difficult in these modulation schemes. Also, since they are not DC-free, these schemes are not used for recording data in the currently-available optical disks, for which the RLL and EFM modulation schemes are the current choices.
RLL (Run Length Limited) is such an encoding scheme that the maximum inversion width Tmax is finite. Especially, an encoding scheme in which the minimum run is d, the maximum run is k and m bits of data are mapped to n modulated bits is called the (d, k, m, n)-RLL encoding scheme. Depending on the parameters selected, there are several different RLL encoding schemes, which can be determined by the performance evaluation parameters indicating the characteristics of the respective schemes.
In the case where the minimum inversion width Tmin providing one of the evaluation parameters is excessively small, a reproduced waveform interference is caused by the OTF (Optical Transfer Function) or the diffraction limit of the optical pickup. The value Tmin, therefore, is advantageously larger.
Conversely, the maximum inversion width Tmax which is related to the ease of self-synchronization is advantageously smaller. More specifically, in order to enable the extracted sync signal to follow the temporal fluctuations (jitter), if any, of the reproduced signal, the signal is required to be inverted frequently. Also, a long inversion interval poses the problem of fluctuations of the DC component.
The detection window width Tw represents the tolerance of the temporal fluctuation (jitter) of the reproduced signal. The playback jitter in the optical disk is considered to derive from various factors. It is not desirable, however, that data displacement, etc. is caused by the temporal fluctuations of the reproduced signal. Therefore, the detection window width Tw is desirably larger.
Table 1 below shows the evaluation parameters of the main encoding schemes.
TABLE 1 ______________________________________ Modulation code evaluation parameters Recording code d k m n Tmin Tmax Tw ______________________________________ NRZI (0) ( ) (1) (1) 1 1 EFM 2 10 8 17 1.44 5.1 0.47 (2,7)RLL 2 7 1 2 1.5 4 0.5 (1,7)RLL 1 7 2 3 1.33 5.33 0.67 ______________________________________
The EFM (Eight to Fourteen Modulation) scheme is one in which eight data bits are converted into 14 modulated bits. Two hundred and fifty six (=28) patterns providing d=2 and k=10 are selected from 214 (=214) patterns. In the process, in order to meet the condition of d=2 between different conversion patterns, one of the coupling bits of three "000", "010", "100" and "001" is selected and inserted to reduce the DC components or the low-frequency components in the final waveform string.
Consequently, this scheme has the feature of small DC components or small frequency components. Insertion of the three connecting bits, however, necessarily reduces the recording density. Also, the algorithm for determining the three bits is considerably complicated and thus complicates the configuration of the modulator.
The (2, 7)-RLL modulation scheme, on the other hand, is generally used for recording data in optical disks. This feature of this modulation scheme lies in that the maximum inversion width Tmax is small while the minimum inversion width Tmin is comparatively large. The disadvantage of this scheme, however, is that since two modulated bits is involved for each one data bit, the detection window width Ts is 0.5 T and is not large, where T is the interval of one data bit. Also, the variable word length makes it necessary to determine the word boundary, leading to the disadvantage of easy error propagation. In order to solve this problem, in the case of recording digital data in an optical disk, a sync signal (RESYNC) is required to be inserted for each predetermined section of data to prevent error propagation.
The (1, 7)-RLL modulation, by contrast, which is an encoding scheme closely watched recently, has the advantages of high density and high data rate with which recording is possible. In this scheme, the minimum inversion width Tmin is 1.33 T and somewhat inferior to the figure 1.5 T for the above-mentioned (2, 7)-RLL modulation. In spite of this, since 2 bits of data are converted into three modulated bits, the detection window width Tw of 2/3 is obtained, indicating a comparatively large margin of demodulation. This scheme, however, lacks the processing for reducing the DC components unlike in the EFM modulation, and still has the problem remaining unsolved.
In recording data in the optical disk, a data bit string is converted into the PLL-modulated bits as described above, and then a recording waveform is produced using NRZ or NRZI. Specifically, the input data string providing the original information are RLL-modulated and, thus by changing Tmin and Tmax, is converted into a PLL-modulated bit string in a form more suitable for the optical disk. A recording waveform is obtained by NRZ or NRZI from this bit string.
The recording scheme using NRZ is called the intermark recording or the bit position recording, as shown in FIG. 6. In the bit position recording, each recording mark corresponds to a modulated bit "1", and the mark position is detected to produce a corresponding bit "1" at the time of playback.
The recording scheme using NRZI, on the other hand, is called the mark length recording or the mark edge recording. In this scheme, the position where the signal level changes, i.e., the leading and trailing edges of a mark correspond to the modulated bits. At the time of playback, the ends of the mark on the disk are detected to produce a corresponding bit "1".
The bit position recording, in which only the presence or absence of the mark is detected, is not easily affected by the disturbance of the mark format (jitter or the like) but has the shortcoming of a low recording density as compared with the mark edge recording in which two modulated bits are recorded per mark. In employing the bit position recording scheme, therefore, a modulated code with a small minimum inversion width Tmin, even with a somewhat small detection window width Tw, is considered suitable for improving the recording density.
The mark length recording scheme has the feature that the recording density can be increased. This scheme cannot be used, however, for some recording media which have marks with different shapes of leading and trailing edges.
Causes of DC level fluctuations!
The signal (reproduced signal) Sin obtained by reproducing the above-mentioned magneto-optical disk, such as shown in FIG. 7A, for example, generally intermittently contains an address component Sa and a data component Sd. Further, in a writable type of disk (such as the magneto-optical disk), the DC level Sdc fluctuates due to an increased light amount required for erasing or writing data.
(DC level fluctuation)
The main cause of the DC level fluctuation is the difference in reflectance between the CD section and the MO section, and it also occurs at the time of erasure. The respective DC level fluctuations will be specifically described taking a rewritable optical disk as an example with reference to FIGS. 8 and 9.
(1) DC level fluctuation between CD and MO sections
Take the magneto-optical disk as an example. As shown in FIG. 8, the magneto-optical signal detector includes an ID (or CD) detection system for detecting bit information and a MO detection system permitting recording and erasure. In the ID detection system, the light amount is changed simply according to the presence or absence of a bit, and the sum A+B of the light received by the light-receiving element A and the light-receiving element B is determined as an ID detection signal (FIG. 9A). In the MO detection system, in contrast, the polarization plane of the beam slightly rotates according to S or N of the direction of magnetization. This slight rotation is detected through the PBS (polarizing beam splitter). The input to the light-receiving element A is increased when the polarization plane rotates in positive direction, while the input to the light-receiving element B increases when the polarization plane rotates in negative direction. The difference A-B is determined as an MO detection signal (FIG. 9B). The ID detection signal and the MO detection signal are generated alternately along time axis. These detection signals are synthesized by being switched alternately thereby to constitute a reproduced signal (FIG. 9C).
In this magneto-optical detector, the difference in reflectance and an optical imbalance between the MO detection system and the ID detection system may cause a difference in DC level between the two detection signals (FIG. 9C). Also, a combination of the ID detection signal and the MO detection signal is intermittently included in sectors, between which the difference in DC level occurs. This difference in DC level makes correct subsequent binarization impossible.
(2) DC level fluctuation at the time of erasure
Further, at the time of ordinary erasure or at the time of automatic erasure of previous information before overwriting, the laser light amount is increased. During the erasure, therefore, the DC level of the ID detection signal produced as a result of detecting bits by the reflected light relatively increases, with the result that the DC fluctuation occurs in the reproduced signal (FIG. 9D).
In the process, the MO detection signal, which represents a difference, is inherently absent. Actually, however, some failure of in-phase rejection due to the variations (such as optical imbalances) between the component elements of the detection system causes DC level fluctuations (FIG. 9E). Also at the time of erasure, the ID detection signal (such as the sector mark or address portion) is required to be reproduced from the reproduced signal including a combination of the ID detection signal and the MO detection signal for specifying the erased portion, and therefore the difference in DC level must be suppressed.
In order to solve the problem of DC level fluctuations, the DC components are suppressed by the AC coupling.
Difficulty of selecting the time constant for AC coupling!
In conventional modulation schemes for recording data such as EFM described above, 0s and 1s are contained at uniform rate. Substantially no DC components are, therefore, contained in the reproduced signal, and no problem was posed to make it difficult to select the time constant for AC coupling.
In the case where DC components are contained in the reproduced signal as in the RLL modulation, however, the mere AC coupling encounters the problem that a correct reproduced signal cannot be read.
Specifically, with the reduction of the time constant for AC coupling, as shown in FIG. 7B, the reproduced signal Sin has a reduced interrupted portion of data including the transient period due to the AC coupling such as the leading portion of the address component Sa or the data component Sd. In spite of the resulting steep rise of each component, the loss of the DC components to be contained in the reproduced signal makes correct reproduction difficult, thereby leading to the problem of deteriorated playback characteristics.
In the case where the time constant for AC coupling is increased, in contrast, as shown in FIG. 7C, the loss of the DC components to be contained in the reproduced signal Sin is prevented. The transient state in the leading portion of the address component Sa and the data component Sd, however, becomes longer in period, and the reproduction becomes difficult of the leading portions (such as the signal component indicated by the circle a) of the reproduced signal Sin. This phenomenon is conspicuous especially in the mark edge recording shown in FIG. 6.
In the case where the reproduced signal is AC-coupled with a predetermined time constant as described above, a small time constant will shorten the period of transient state and causes the loss of the DC components inherently required of the reproduced signal, thereby making correct reproduction difficult. With an excessively large time constant, on the other hand, a longer period of transient state results. In such a case, although the DC components of the reproduced signal are not lost, the reproduction becomes difficult of the leading portion of the address component Sa and the data component Sd.
In order to realize a high data density and a high bit rate, the use of a modulation scheme such as (1, 7)-RLL is considered preferable. The (1, 7)-RLL modulation, unlike the EFM modulation, does not take into consideration the suppression of DC components. Take the above-mentioned problem as an example. As long as a modulation scheme such as the (1, 7)-RLL modulation is employed which inherently contains DC components, data may not be read by the mere suppression of the DC components by AC coupling.
More specifically, the (1, 7)-RLL modulation has a minimum run of d=1 and a maximum run of k=7 with at least a 0 inserted between a 1 and a 1 and a maximum run of seven 0s. Assume that the AC coupling is carried out with a comparatively short time constant to quickly reproduce the sync data included in the leading portion of each sector. In the case where comparatively long runs successively appear in the immediately-following data portion to be reproduced, the DC components inherently contained in such a portion are lost. Proper binarization thus becomes impossible in subsequent stages.
In the case where the AC coupling is effected with a comparatively large time constant to properly reproduce the DC components inherently contained in comparatively long successive runs, on the other hand, the transient state in the reproduction of sync data continues for a long time in the leading portion of the sector. Correct reproduction of the sync data therefore becomes impossible during such a period.
The present invention has been developed in view of the above-mentioned problems, and the object of the invention is to provide a playback circuit with a simple configuration which can prevent the loss of the DC components to be inherently contained in the reproduced signal and at the same time can shorten the period of transient state in the interrupted data portion.